Structural and magnetic phase transitions in Ca$_{0.73}$La$_{0.27}$FeAs$_2$ with electron overdoped FeAs layers

TL;DR

This study investigates the structural and magnetic phase transitions in Ca$_{0.73}$La$_{0.27}$FeAs$_2$, revealing a monoclinic to triclinic transition, a paramagnetic to AFM transition, and unique Fermi surface features including an extra electron pocket.

Contribution

It provides new insights into the phase transitions and electronic structure of Ca$_{0.73}$La$_{0.27}$FeAs$_2$, highlighting its unique magnetic and Fermiology properties.

Findings

Monoclinic to triclinic transition at 58 K

Paramagnetic to AFM transition at 54 K

Presence of an extra electron pocket with As chain character

Abstract

We report a study of the Ca$_{0.73}$La$_{0.27}$FeAs$_2$ single crystals. We unravel a monoclinic to triclinic phase transition at 58 K, and a paramagnetic to stripe antiferromagnetic (AFM) phase transition at 54 K, below which spins order 45$^\circ$ away from the stripe direction. Furthermore, we demonstrate this material is substantially structurally untwinned at ambient pressure with the formation of spin rotation walls (S-walls). Finally, in addition to the central-hole and corner-electron Fermi pockets usually appearing in Fe pnictide superconductors, angle-resolved photoemission (ARPES) measurements resolve a Fermiology where an extra electron pocket of mainly As chain character exists at the Brillouin zone edge.